1. Field of the Invention
The present invention relates to a current sense circuit and method for a bridgeless boost (BLB) PFC circuit using a single current transformer.
2. Background Art
The bridgeless boost PFC has proved to be a good alternative topology for a power factor correction circuit, as shown in FIG. 1. Compared with the conventional PFC circuit using a bridge, shown in FIG. 2, the bridgeless PFC circuit reduces the number of semiconductors in the conduction path. For the conventional PFC, there are three semiconductors in the current conduction path. As for the bridgeless PFC, only two semiconductor devices are in the conduction path at any time. Considering that both of the circuits work as a boost DC/DC converter, the switching loss should be the same. Therefore, the bridgeless PFC can reduce the circuit conduction loss and improve the circuit efficiency. Further, since the circuit only uses two MOSFETs and two diodes, as compared with the conventional PFC, which has one MOSFET and five diodes, the circuit is much simplified.
Although the bridgeless PFC circuit of FIG. 1 has a simplified circuit structure and improved efficiency, the issue of current sensing is problematical.
For the conventional PFC circuit of FIG. 2, the inductor current can be sensed through a shunt resistor placed in series in the return path of the inductor, as shown in FIG. 3. Thus the current signal is transformed into a voltage signal which can be used for control purposes. However, for the bridgeless PFC circuit, the inductor current return path is at the AC side, as shown in FIG. 4. Normally the control circuit has a common ground with the output. Therefore, an isolated current sensing method is required for the bridgeless PFC.
To achieve isolated current sensing, a 50 or 60 Hz current transformer gives a straightforward solution, as illustrated in FIG. 5. However, since the low frequency current transformer will cause a large phase different between the input and output signals, using the sensed signal to control the power factor correction circuit will impair the power factor. Further, the low frequency transformer is large, heavy and also expensive, thus it cannot be accepted for kilowatt-range power supplies.
Another isolated current sensing method is to use a differential mode amplifier, as shown in FIG. 6. The differential mode amplifier doesn't have a phase difference between the input and output. It can give a good control signal. However, since the bridgeless PFC operates at a high switching frequency and a high output voltage, the common mode voltage in the differential mode amplifier will generate noise on the sensed signal. Considering that the sensing voltage is kept low to minimize the power loss in the shunt resistor, the noise caused by the common mode voltage can distort the sensed current. Further, since the differential mode amplifier is expensive and requires an extra power supply, it is not a practical solution either.
Another current sensing method is high frequency reconstruction, as shown in FIG. 7. In this current sensing scheme, two current transformers T1 and T2 are in series with S1 and S2. In each half line cycle, one of them is saturated and gives no output signal and the other one gives the switching current signal. The current transformer T3 is able to sense the diode current. By adding the switch currents together with the diode current through the high frequency current transformers, the inductor current can be sensed. Therefore, a total of three current transformers are required for the control circuit. Even using peak current mode control, wherein only the switching current is required for the power factor correction control, at least two current transformers are still required.
Because of these drawbacks, although the bridgeless PFC circuit has existed for around 20 years, it still hasn't been accepted by the industry. Not only does the circuit suffer from a severe EMI noise problem, it also has the issues of voltage sensing and current sensing. Therefore, in the past 20 years, most of the work has been directed to solving the control issues of the circuit. One attractive method has been shown to improve the current sensing and voltage sensing issues of the bridgeless PFC circuit. The circuit schematic is shown in FIG. 8.
In FIG. 9, (a) and (b) are the respective equivalent circuits of the circuit operating in the positive and negative half line cycles. In each half line cycle, the bridgeless PFC circuit works as a boost DC/DC converter. The whole circuit is equivalent to two boost circuits added together. All the inductor current will go through the shunt resistor, where the inductor current can be sensed and can be used for the control circuit.
Although this circuit can provide good current sensing in the bridgeless PFC circuit, it still has several constraints:                Two extra diodes need to be used.        The two extra diodes require an extra heat sink, which makes the circuit even more expensive.        By using the shunt resistor, extra power loss is introduced in the circuit.It would be desirable to reduce the component count, cost and current sensing loss.        